Technology that continuously analyzes off-gas chemistry is an important tool for optimizing, controlling and improving the performance of combustion processes such as electric arc furnace (EAF) and basic oxygen furnace (BOF) steelmaking processes or the like.
In the EAF steelmaking process, full-spectrum off-gas analysis for CO, CO2, H2, O2, H2O vapor and N2 is a valuable tool for holistic optimization and control of the steelmaking process.                N2 analysis is effective for assessing and dynamically controlling fume system suction to avoid both over and under drafting conditions        CO, H2, O2 & N2 analysis are effective for determining if the EAF is operating in an overly oxidizing or reducing atmosphere        CO, CO2 & H2 analysis are effective for optimizing and dynamically controlling burners and for optimizing the charge carbon practice        CO & CO2 analysis are effective for optimizing and dynamically controlling carbon injectors        CO, H2 & O2 analysis are effective for optimizing and dynamically controlling the oxygen lances        H2 & H2O vapor analysis are effective to determining the onset of a water panel leak into the furnace        CO, CO2, H2, O2, H2O vapor and N2 analysis is required to close a real-time mass & energy balance for the EAF process        
Similarly, in the BOF steelmaking process having a full spectrum off-gas analysis for CO, CO2, H2, O2, H2O vapor and N2 is preferred to close a real-time mass & energy balance for the BOF process which is effective for controlling the efficiency of the oxygen blowing practice, controlling the amount and the timing of post combustion oxygen flow and determining when to terminate the oxygen blow because the aim steel carbon and temperature endpoints have been achieved.
To date, continuous off-gas analysis technology for industrial applications has remained essentially unchanged since about 1997 being based on one of two principle methods;    1. Extractive systems use a vacuum pump to continuously extract a sample of process off-gas through a probe positioned in the fume duct with said probe connected to a hollow often heated conduit that directs the off-gas sample to a continuous gas analyzer. E. J. Evenson U.S. Pat. No. 5,777,241 describes such an extractive system for optimization and control of steelmaking processes. Depending on the gaseous species to be analyzed, various analytical methods are employed with extractive technology including mass spectrometry which can analyze most gaseous species, non-dispersive infra-red (NDIR) which is a standard method for analyzing CO and CO2, a solid state electrochemical cell and thermal conductivity which are standard methods for O2 analysis and for H2 analysis respectively.    2. In situ laser systems transmit a single beam or a combined beam or multiple individual beams within the visible, near and mid IR range through the off-gas as it flows in the fume duct for subsequent pick-up by an optical detector(s). D. K. Ottesen U.S. Pat. No. 5,984,998 and S. C. Jepson U.S. Pat. No. 6,748,004 present examples of in situ laser systems for measuring off-gas chemistry. In general, the transmitted lasers wavelength is modulated around the particular spectroscopic line of the gaseous species of interest. The amount of absorption in the detected beam is subsequently used to calculate the concentration of that particular species in the off-gas. Multiple lasers are required depending on the gaseous species to be analyzed, typically one near IR range laser with a suitable wavelength for CO2 and H2O vapor, a second near IR range laser with a suitable wavelength for CO and a third visible range laser with a suitable wavelength for O2. It is noted that three separate lasers of the correct wavelength are required to analyze CO, CO2 and O2. Because the CO and CO2 absorption peaks begin to overlap as the off-gas temperature increases above about 300° C., in situ laser systems need to employ one near-IR range laser with a suitable wavelength for CO2 and a separate second near-IR range laser for CO. A third visible range laser with a suitable wavelength is required for O2. The in situ method can also utilize either the CO or CO2 laser to analyze H2O vapor if required. Because varying amounts of particulate matter are present in most industrial process off-gas, there is the possibility that the laser beam will suffer from attenuation which will scatter or block the beam. In many industrial applications, said attenuation problems can be reduced but not completely eliminated by employing two horizontal or vertical probes that are continuously purged with an inert gas such as N2 with one probe housing the laser beam emitters and the second probe housing the laser beam detectors. These two probes extend into the fume duct from opposite sides with one probes open end being in close proximity to the opposite probes open end which serves to reduce the path length that the beams must successfully traverse between emitter and detector and minimize laser beam attenuation problems associated with particulate matter interference.
Extractive and in situ laser technologies each have their respective advantages and disadvantages and hence neither technology provides a complete off-gas analysis solution;                Analytical Capabilities: Extractive off-gas systems have the advantage of being able to utilize and combine a range of analytical methods to provide a virtually complete spectrum of off-gas chemistry. For example, steelmaking off-gas chemistry consists almost exclusively of six gaseous species which vary in concentration according to process dynamics; CO, CO2, O2, H2, H2O vapor and N2. For all practical purposes and unless a foreign gas is deliberately introduced into the furnace atmosphere, the concentration sum of these six gaseous species totals 100%. As explained earlier, various extractive analytical methods can be used to analyze for CO, CO2, O2, H2 & H2O vapor. In the case of N2, it can either be analyzed by extractive mass spectrometry or it can be calculated with reasonable precision by summing the analysis of the remaining five principle gaseous species and subtracting from 100%.         By comparison in situ laser systems can use a combination of lasers in the correct wavelength range to analyze specific gaseous species of interest. For example, for in situ analysis of high temperature off-gas such as for steelmaking applications, three separate lasers of different wavelengths will be required to analyze CO, CO2, O2 & H2O vapor. However, in situ laser technology is not technically capable of analyzing many mononuclear diatomic gases including N2 and H2 (S. Schilt, F. K. Tittel and K. P. Petrov, “Diode Laser Spectroscopic Monitoring of Trace Gases”, Encyclopedia of Analytical Chemistry, pages 1-29, 2011). Hence, compared to extractive methods in situ laser technology has the disadvantage of limited analytical capabilities.        Analytical Precision: Extractive systems can tailor their analytical method to meet the analytical precision needed for specific industrial process control situations. Hence, extractive technology has the advantage of having the flexibility to tailor the analytical precision to the application requirements.         The analytical precision of laser systems is gas species dependent. The amount of absorption of the beam determines the analytical precision. Each gaseous species has an optimum beam path length that provides the optimum amount of absorption and the optimum analytical precision. In general, using a path length with the optimum absorption will meet the analytical precision needed for many industrial process control situations. However, path lengths that are shorter than the optimum will reduce the amount of absorption and the analytical precision. Conversely, too long a path length can result in signal saturation and limit the measurement span of the instrument. In situ lasers use a fixed path length defined as the distance between the laser beam emitter and detector. This fixed path length is common to all gaseous species being analyzed. As described previously, in situations where there are optical signal transmission difficulties due to beam attenuation in dusty industrial off-gas environments, in situ laser systems select the fixed path length to minimize laser interruptions by positioning two opposite facing inert gas purged probes. The separation distance between the open ends of said two probes defines the fixed path length that the laser beam must transmit through the process off-gas. Hence, compared to extractive methods which can be designed for high analytical precision for all gaseous species, the fixed, common path length used in in situ laser technology may or may not provide the required analytical precision for all gaseous species being analyzed.        Calibration: Most extractive analytical methods require periodic recalibration to compensate for analytical drift. Depending on the gases to be analyzed, extractive systems can require several specialized calibration gases which can be expensive. Hence, extraction technology has the disadvantage that the analytical methods require periodic recalibration and specialized calibration gases.         In situ laser systems are often equipped with reference cells that contain known concentrations of the gaseous species being analyzed. Laser technology uses the known reference cell gas composition to self-calibrate the system. Hence, compared to extractive methods, in situ laser technology has the advantage that it does not require periodic recalibration or specialized calibration gases.        Analytical Response Delay: The analytical response delay for extractive system depends on the residence time of the off-gas sample from the probe tip to the analytical cells located in the analyzer. The residence time is dependent on the volume of the gas train (probe, transport conduit & filtration system), the extraction flow rate of the gas and the physical distance between the probe and the analyzer which is often longer than desirable because of the need to house the analyzer in a large, environmentally protective enclosure. While extractive systems can use a high velocity pump to rapidly extract off-gas at high flow rate through the probe, often the analytical devices inside the analyzer are designed to use only a slower velocity gas flow rates and therefore the majority of the off-gas extracted sample is vented before the analyzer which uses only a slower velocity slip stream. All of these factors serve to increase the analytical response delay of extractive systems. Most modern extractive systems for example those used in the steel industry are designed to provide an analytical response within about 20 to 40 seconds from the time the gas enters the probe tip until the corresponding gas analysis is reported. In situ laser systems have a much shorter response delay of the order of 2 seconds because the off-gas is not physically transported to a remote analyzer. Hence, compared to extractive methods in situ laser technology has the advantage of a much shorter analytical response delay.        Analytical Reliability: Extractive off-gas systems can be categorized as “active” technology. Typically the extractive analysis system is interfaced with the furnace control network so that whenever the industrial process is producing off-gas, the extractive system automatically switches on a pump or the like to provide high suction to actively extract a sample of off-gas through the probe which is appropriately positioned in the fume duct. The off-gas sample is transferred at high flow rate through a hollow heated or unheated conduit to the analyzer. For dirty, humid off-gas as exists in many industrial processes, the hot, humid off-gas sample is first passed through a series of progressively finer filters to remove particulate matter from the off-gas sample. Since many analytical techniques mentioned previously require clean, dry gas for reliable chemical analysis, after filtration the process off-gas is typically passed through a condenser or the like to remove water vapor prior to analysis which is subsequently reported on a dry basis. In a few select situations such as when it is necessary to avoid formation of corrosive acids in the condensate or when analyzing some specific gaseous species such as water vapor, the cleaned off-gas sample maybe kept at a temperature above its dew point and analyzed wet. However, in such instances the analytical cells must be designed to operate reliably and precisely at elevated temperature. Extractive systems are typically designed to automatically and periodically switch to an active back-purge for example during periods when the industrial process is not producing off-gas. This automatic back-purge can consist of high pressure compressed air or inert gases such as N2 or the like and are designed to clean the probe and filters of accumulated particulate matter. Historically, such extractive technology that alternates between positive suction and back-purging has demonstrated exceptional analytical reliability, for example when properly maintained, extractive technology applied for in harsh steelmaking process conditions has reportedly demonstrated better than 99% reliability to provide continuous off-gas chemistry from start-to-end of the steel producing heat.         By comparison, in situ laser systems can be categorized more as “passive” technology that relies on passive transmission of laser beam(s) through the off-gas fume from an emitter to a detector. Attenuation of the laser beam that prevents a sufficient level of detection will result in interrupted off-gas analysis. For example, under steelmaking process conditions, early in situ laser systems suffered from serious laser beam attenuation difficulties and lost signals because of significant amounts of dust prevalent in the harsh process off-gas. As discussed previously, various methods have been reported to reduce attenuation difficulties including the use of continuous inert gas purged, opposite facing horizontal or vertical probes to shorten the path length that the laser must successfully transmit through the dirty process gas, or, particulate deflectors or impingers such as disclosed by W. A. Von Drasek U.S. Pat. No. 6,943,886. While these devices have considerably reduced beam attenuation problems compared to original full path length in situ designs, because of the passive nature of laser transmission there still remains a risk that one or more of the in situ laser beams may suffer from periodic and unpredictable interruptions in signal transmission especially when dust loading is particularly high. For example, steel industry reported information indicates that on average about 50% of EAF heats will experience some degree of lost laser signals due to fume signal interruption. Any lost laser signals during EAF scrap melting would limit effectiveness of off-gas water leak detection systems during critical melting periods when hung-up scrap can fall into bath and create a metal slosh event that can trigger a water leak related explosion. In addition laser signal interruption limits the effectiveness of process monitoring and control functions. Hence, compared to extractive methods in situ laser technology has the disadvantage of uncertain analytical reliability especially in harsh industrial situations such as steelmaking processes.        Installation and Maintenance Considerations: Most extractive analyzers used in harsh industrial situations must be housed within a protective room or enclosure that ensures the electronics are maintained within an acceptable working environment particularly regarding minimizing industrial dust and maintaining suitable ambient temperatures. If a suitable enclosure does not already exist within the plant, a protective room will need to be constructed which adds to the cost of installation. To minimize analytical response delays, the protective analyzer room needs to be located within close proximity (usually with ˜30 meters) from the extraction probe. Depending on the particular circumstances, finding a suitably sized area in close proximity to the probe can be challenging in confined industrial spaces. Because extractive systems filter and usually dry the process off-gas prior to analysis, extractive systems require regular maintenance to inspect and replace clogged filters, to inspect and service pumps and condensers as well as discussed previously, to periodically check and adjust calibration to ensure analytical precision.         By comparison, in situ laser systems mount the laser beam emitters and receivers on the fume duct often inside protective path length shortening probes as discussed previously. The laser beam is usually transmitted to the emitter from a remotely located laser by fiber optic cable. The received signal after the beam has passed through the process off-gas is also transmitted electronically. As such, since the off-gas does not physically transfer to the lasers and signal analysis componentry, it can be located remotely without distance restrictions. In addition, in situ systems do not require filters, condensers or pumps. Hence, compared to extractive methods in situ laser technology has the advantage of lower installation costs and less maintenance requirements.        Process Control Functionality: The functionality of the off-gas analysis technology for optimizing, controlling and improving the performance of a combustion process will depend largely on the analytical capabilities of the off-gas analysis system. For example, the following table provides the key gaseous species analyses required to provide complete process control and optimization functionality in a steelmaking furnace. Hence applicant has recognized the extractive methods which provide a complete off-gas analysis spectrum have the advantage over the limited analytical capability provided by in situ laser technology.        